The present invention relates to a square bag made of thermoplastic film and a machine for manufacturing said bag.
In the case of general square bags having a tubular body which is made of thermoplastic film such as polyethylene film and is flat when empty, said tubular body is commonly prepared by the inflation method and is inferior in ink-adsorbing property as it is so that it is impossible to perform a satisfactory printing thereon. Therefore, in order to improve said ink-adsorbing property of such bags, it is usual to apply corona discharge of a high-voltage electricity to the outer surface thereof.
Besides, such bags as above are ordinarily prepared by welding closures made of thermoplastic film onto the outer surface of both the top and bottom openings of said tubular body by heat-sealing.
However, the outer surface of said film subjected to treatment with corona discharge as above loses the affinity with oil inherent in polyethylene and becomes hydrophilic. Accordingly, when two outer surfaces subjected to such treatment alike are heat-sealed with each other, the intercrossing of polyethylene molecules of the welded portions is impeded and said two surfaces are apt to come off simply by the sealed portion when subjected to tension after cooling. Futher, even when a surface subjected to treatment with corona discharge and a surface not subjected to treatment with corona discharge are heat-sealed each other, the welding strength is very weak, entailing a defect that the product bag is apt to open while in use.
With a view to elimination of such defects, it has so far been proposed to scrape off the thus treated surface by a portion required for welding by the use of an edged tool, a file or a wire brush. But, this measure is defective in that, it is very difficult to scrap exclusively the treated portion off the surface without causing damages of the film as a whole, and it fails to produce a satisfactory bag.
Further, it has been proposed to use an adhesive having high adhesive power such as the rubber-type adhesive instead of resorting to heat-sealing in order to avoid the foregoing drawbacks. However, this mreasure is also defective in that it requires preparation of a special adhesive therefor, which means an additional cost of the product bag, as well as a special apparatus for the purpose of drying the adhesive used, which means requirement for an additional time of processing.